It depends upon a lot of things but mostly it depends upon geometry. Both the aircraft must be properly aligned. When the missile loses track on the first aircraft, the second aircraft must be in the seeker field of view or very close to that field of view. The seeker is not given unlimited authority to go find another target. It is constrained to try to re-acquire the original target not go off on some wild goose chase just looking for any old target.
Some of the newer missiles are designed to avoid jamming techniques by firing them into an intercept vector without active radar control and then allowing the seeker to acquire the target when it is too close for jamming to be effective.
It seems logical that such a missile might find it more difficult to acquire a smaller more agile target like an SU-25 and instead acquire a larger, more stable target like a passenger plane.